1. Field of the Invention
Digital transmission systems contain network nodes, controlled by a network management system, with interfaces for digital signals of the plesiochronous digital hierarchy PDH and for STM-1 signals of the synchronous digital hierarchy SDH.
2. Description of the Related Art
FIG. 1 shows a multiplex structure of the European Telecommunications Standards Institute ETSI, Recommendation ETS DE/TM-301, version 1.2 of 30.06.1990. In this structure, AU means administrative unit, C is container, STM is synchronous transport module, TU is tributary unit, TUG is tributary unit group and VC is virtual container. The numbers on the lines specify how often a similar unit per transport module STM-1 to be formed is transmitted. The arrows indicate the direction of transmission during multiplexing; the signal flow is reversed during demultiplexing. The tributary unit TU-11 is shown dashed because it originates from a multiplex structure according to FIG. 1.1 of CCITT Recommendation G.709 of 6.6.1990.
In the printed Siemens document "NK2000 Network Node", issued by the Transmission Systems Department, POB 70 00 71, D-W-8000 Munich, order No. S42022-D4200-A1-3-29, a network node is described which contains a network control unit NCU and various switching devices connected to it and to one another by means of buses.
A CCM64K switching device is a cross-connect multiplexing device with a switching network for non-blocking routing of 64-kbit/s signals and with interfaces for 2-Mbit/s signals.
A CCM2 switching device is a cross-connect multiplexing device which contains a multi-stage synchronous switching network. It provides for non-blocking routing at various switching levels of the synchronous digital hierarchy SDH. These are the switching level TU-12 for s-Mbit/s signals and the switching level TU-3 for 34- and 45-Mbit/s signals.
A CC155 switching device is a cross-connect device with a synchronous switching network for non-blocking routing of 140-Mbit/s signals and STM-1 signals of a bit rate of 155 Mbit/s at the AU-4 switching level.
A network node concept DBPT of the firms ANT Nachrichtentechnik GmbH and Standard Elektrik Lorenz AG contains a switching device, corresponding to the CCM2 switching device, with an OH server controlled by the network control unit, which receives the section overheads of all incoming STM-1 signals and generates corresponding section overheads for the STM-1 signals to be output.
FIG. 2 shows the frame of a synchronous transport module STM-1 with 270 columns SP or bytes and 9 rows R. This form was chosen in order to be able to represent the frame with a length of 9 * 270 Sp on one DIN-A4 sheet. For system-internal purposes, the frame contains a regenerator section overhead RSOH, a multiplexer section overhead MSOH and an administrative unit pointer AU PTR.
FIG. 3 shows the section overhead of the STM-1 frame according to FIG. 2 with a byte allocation according to FIG. 5.2 of CCITT Recommendation G.708 of June 1990. The supplementary bytes of the RSOH and MSOH section overheads are used, for example, for synchronising to the beginning of the frame (A1, A2), for determining the bit error rate of the regenerator section (B1) and of the multiplex section (B2), for identifying the STM-1 signal (C1), for transmitting control information items (D1-D12), as engineer's order channel (E1, E2), for automatic stand-by switching (K1, K2) or as spare (Z1, Z2). The F1 byte is available to the user. These supplementary bytes are distributed over seven columns Sp of the frame.
From European Patent Application EP-0,407,851 A2, a process for switching multiplex signals through via a cross-connect device is known in which data, arranged in data blocks, of the plesiochronous or synchronous digital hierarchy of various multiplexing levels are converted into uniform cross-connect data blocks. These have a bit rate of 38.912 Mbit/s, are accommodated in frames in 152 rows and 16 columns and are designated as D39 signals. Their circuit implementation is described in a prior proposal (P 39 30 007.2).
According to FIG. 1, up to sixty three 1.5- or 2-Mbit/s signals, three 34- or 45-Mbit/s signals or one 140-Mbit/s signal can be multiplexed into one STM-1 signal via the tributary units TU-12. Such a signal is divided into four D39 signals in the receiving section of an interface unit of the CCM2 switching device and are supplied to the space-division switching matrix after a time-division switching stage in which the up to sixteen TU-12 or D2/S signals contained in a D39 signal can be arbitrarily distributed in time. For this space-division switching matrix, the number of D39 signals must be doubled between the time-division and space-division switching network, that is to say eight D39 signals must be provided in the CCM2 switching device. Since sixty four TU-12 signals can be transmitted in the four D39 signals, one column is not occupied.
When the tributary unit TU-11 according to FIG. 1 is used, the multiplexing structure shows that, instead of sixty three 1.5-Mbit/s signals, eighty four of those signals are multiplexed for the US hierarchy. After doubling, this requires twelve D39 signals.
If 34-, 45- or 140-Mbit/s signal bursts are conducted via the space-division switching network, the time-division switching function and thus the doubling of the D39 signals is omitted for these so that more free capacity is available for the transmission of the supplementary bytes in these cases.
Processing of a complete RSOH and MSOH section overhead requires considerable effort which must be carried out for each STM-1 signal in a CCM2 cross-connect device.